A stepper motor is an electromechanical device which rotates a discrete step angle when energized electrically. Stepper motors contain a rotor that has a permanent cylindrical magnet with many poles around its circumference. The rotor rotates inside two sets of stator coils, each of which has a row of metal teeth. When an electric current is transmitted to a stator, each north pole on the rotor lines up with an overlapping pair of south poles on the stator teeth, while each south pole on the rotor lines up with a pair of north poles on the stator teeth. When the current transmitted to the stator is reversed, each pole on the rotor shifts by one stator tooth.
The step angle is fixed for a particular motor and thus, provides a means for accurately positioning in a repeatable uniform manner. Typical step angles vary from as small as 0.72 degrees to as large as 90 degrees. The stepper motor starts and stops in discrete angular excursions of uniform moves so that the output steps are always equal to the number of input pulses.
Small stepper motors have been utilized to drive a set of camera shutter blades. The number of pulses transmitted to the stepper motor determines the aperture achieved by the shutter blades. Such shutter blades are reasonably fast and simple to drive electronically. The prior art utilized carefully tailored pulses to generate controlled wave forms. The carefully tailored pulses help to assure that the shutter blades achieve its aperture position as quickly as possible. Stepper motors, however, are notorious for erratic performance due to inertia effects, temperature friction, and a host of other subtle phenomena.
The prior art attempted to solve the temperature problem by trying to drive constant current into the stator coils over the desired temperature range. In some cases the above is useful in reducing the variations in the torque of the stepper motor due to temperature variation. However, the above does not eliminate the effect friction has on the performance of the stepper motor. Usually the only thing that can be done about the variation of friction with temperature is to try to eliminate the friction from the system by using low friction materials wherever components rub.
Problems To Be Solved By The Invention
One of the disadvantages of prior art shutter blades were that the shutter blades were sensitive to environmental effects, i.e., temperature. Cameras are expected to operate reliably over a temperature range of 0 degrees F. to 120 degrees F. Temperature variations cause many problems with stepper motors that control shutter blades. When the temperature drops the conductivity of copper in the stator coil goes up and the magnetization of the rotors magnets goes up. Thus, the above phenomena will effect the aperture achieved by the shutter blades.
Another problem of the prior art was that the proper operation of the existing stepper motor driven shutters are dependent on a finite amount of friction in the system. In other words the shutters work well at room temperature but not over a broad temperature range. The primary contributor to the above problem is the change of friction with temperature.
Another problem of the prior art was that some stepper motor control systems were large and complex and required accurate positional information about the stepper motors rotor to control the stepper motor.
An additional problem of the prior art is that existing methods of controlling stepper motors are large and complex and not readily usable to control the shutter blades of a camera.